OpenOpcUa

OpenOpcUa is an opensource and multi-platform C/C++ project

OpenOpcUa formely Open Source for OpcUa

First remember that Open is different than Free.
OpenOpcUa is an initiative launched in 2009 by a consortium of
international
companies led by Michel Condemine (4CE Industry). The
objectives of this consortium are multiple:

Create a professionnal quality codebase implementing OpcUa concept

Help users understand OpcUa technology

Today OpenOpcUaCoreServer is the only one Open Source OPC UA server certified by the OPC Foundation Compliance Test Tool.